A battery pack which is formed by uniting a circuit substrate having a charge/discharge protection circuit with a battery is required to have a compact structure, low connection resistance, high reliability of connected parts, and high production efficiency.
Conventional battery packs are structured such that the battery is accommodated inside a case with a protection element and a circuit substrate. To give an example, there is known a battery pack wherein a battery is accommodated in a case having a terminal window, and a protection element and a circuit substrate are arranged in a space provided between the battery and the case. A lead plate that connects the battery with the circuit substrate is arranged opposite the terminal window to serve as an external terminal (see, for example, Prior Art Document 1).
Another known battery pack uses a terminal holder having a fitting protrusion at the bottom that fits into a recess on a battery that is formed by the protruding periphery of one end face of the battery. A protection element is set inside the terminal holder, and a terminal plate is secured to the top face of the holder. After the terminal holder is coupled to the battery with an adhesive tape or heat shrink film, a lead plate that will extend to the outside is welded to the terminal plate and the battery case, and the holder and the battery are either accommodated in a case or covered by a heat shrink tube or the like to constitute a battery pack (see, for example, Prior Art Document 2).
In yet another known battery pack, the sealing plate of the battery includes a connection boss that is coupled to the circuit substrate so as to connect the battery and the circuit substrate both mechanically and electrically (see, for example, Prior Art Document 3), which is contrary to the conventional practice of connecting the battery and the circuit substrate mechanically and electrically with a lead plate, with the circuit substrate being temporarily held on the battery using a holder.
In another known battery pack, the battery is partly or entirely insertion molded such that an external connection terminal is exposed to the outside, and a resin mold in which a protection element is fixedly set is formed in one piece with the battery at its one end (see, for example, Prior Art Document 4).
Prior Art Document 1: Japanese Patent No. 3244400
Prior Art Document 2: Japanese Patent Publication No. 2000-243362
Prior Art Document 3: Japanese Patent Publication No. 2002-298809
Prior Art Document 4: Japanese Patent Publication No. 2004-95329
In the structure shown in Prior Art Document 1, however, because there is provided a space between the case and the battery for placing the protection element and the circuit substrate, the case is large. While there is the risk that the circuit substrate or protection element may be damaged by vibration, it is hard to retain them in position to the battery or case, and high reliability is hard to achieve. Further, the connection resistance is high because of the use of a lead plate.
In the structure shown in Prior Art Document 2, a protection element is securely attached to the battery through the terminal holder. A lead plate is welded to the assembly of this battery and holder, which is then accommodated in a case to form the battery pack. Therefore the battery pack is large in size and has high connection resistance. Also, it requires a large number of assembling process steps, because of which high productivity is hard to achieve.
In the structure shown in Prior Art Document 3, the circuit substrate is secured to the battery using a special sealing plate that has a connection boss so that connection is established without using a lead plate to reduce the connection resistance and the number of assembling process steps. However, these circuit substrate and battery are then accommodated in a case to form the battery pack and therefore it is large in size.
In the structure shown in Prior Art Document 4, the battery, the protection element, and the output terminal are insertion molded so that the protection element and the output terminal are secured in the resin mold. With this structure, the number of components is fewer and the battery pack is compact, but insertion molding presents other problems such as high equipment cost and difficulties in achieving high dimensional precision and high productivity.
Accordingly, in view of the above-described problems in conventional techniques, it is an object of the present invention to provide a compact battery pack having low connection resistance, high reliability, and high productivity.